1. Field of the Invention
For a number of years, there have been available to consumers transparent, flexible films of thermoplastic organic resins packaged in rolls, which films are used for the wrapping and preservation of various foods in home refrigerators. It is to the improvement of the surface characteristics of such films that the present invention is directed.
The flexible thermoplastic film wrappings which have achieved commercial importance and which are modified according to the present invention include those manufactured from plasticized polyvinyl chloride, copolymers of polyvinylidene chloride, e.g., Saran, polybutylene and particularly low density polyethylene and copolymers thereof. Such films offer the advantages of transparency, low water vapor transmission, high permeability to oxygen, and resistance to mechanical and chemical deterioration over long periods of time even when subjected to substantial temperature changes as by refrigeration. Moreover, such films may be utilized a number of times in wrapping and unwrapping of food products without significant loss of the desirable mechanical and physical characteristics, such as self-adherence, strength and flexibility.
Notwithstanding the generally desirable properties of such thermoplastic film wrappings, which have to a substantial extent displaced other types of protective wrappings, such as wax paper, plastic coated paper, aluminum foil and the like, there is one property of importance to the householder which it has long been desired to improve, namely, the cling of the thermoplastic film. It is the specific object of the present invention to increase the cling of household thermoplastic food wrapping while at the same time maintaining other desirable properties of the film and, indeed, even improving such properties while increasing the cling value.
As utilized herein, the term "cling" is used to indicate the shear strength cling of the film surface to such other smooth surfaces as glass, stainless steel, aluminum, and plastic articles, such as bowls to which adherence of household food wrap is highly desired in conventional use by the householder wherein bowls of such smooth surfaced materials are used to contain food. The bowls are covered by adherence of a sheet of food wrap over the bowl and over a part of the outside surface thereof.
Of great significance in the present invention is the fact that improved cling is obtained without sacrifice of the other desirable properties of the food wrap by virtue of a coating which does not impart excessive stickiness or display a greasy feel brought about by the use of other coatings previously disclosed in the prior art.
2. Description of the Prior Art
In the past, numerous efforts have been made to modify the surface characteristics of transparent flexible thermoplastic films, most of which efforts have been directed to films used for commercial packaging of various food products. One problem which has frequently been addressed is the problem of fogging encountered when the normally hydrophobic thermoplastic films are utilized to package moisture emitting products. Examples of patents dealing with this problem are U.S. Pat. Nos. 2,365,297; 3,048,263; 3,212,909; 3,205,077; 3,355,313; and, 3,741,803. A common ingredient in many of the patents, wherein a coating is applied to the surface of the thermoplastic films, is a surface active or wetting agent which improves the spreading action of the various coating compositions on the films which films are otherwise resistant to wetting by liquids. Indeed, the alkali metal dialkyl sulfosuccinates which form the essential ingredient of compositions employed in the present invention (as described in more detail hereinafter) have been utilized as wetting agents in various compositions. In addition, alkali metal salts of dialkyl sulfosuccinates have, themselves, been found to have an anti-fogging effect on thermoplastic films. Thus, in French Pat. No. 1,321,800, issued Feb. 11, 1963, said alkali metal salts of dialkyl sulfosuccinates are described as agents for anti-fogging purposes where these salts, without other ingredients, are applied to the thermoplastic films in a solvent such as water or a lower aliphatic alcohol. No mention of cling is found in this patent, and, indeed, the antithesis of cling is obtained since the inventor describes the films produced as being non-sticky, possessing good sliding properties and great resistance to scratching. Experiments by the present inventor have confirmed these observations, where it has been found that the sodium dialkyl sulfosuccinates deposited from an alcohol solution result in a hard, non-cling surface.
Though most of the prior art deals with modification of surface characteristics of films used in commercial applications, U.S. Pat. No. 3,070,462 to McConnell et al is also concerned with flexible film wrapping of the nature herein contemplated, namely, those intended for household use. This patent alleges the improvement of a number of characteristics of the film, including reduction in blocking and electrostatic propensities and further alleges improved contact adhesiveness for smooth surfaces, i.e., cling, by application of a surface coating comprising a liquid surface modifying agent selected from the group consisting of aliphatic monohydric alcohols having in excess of 6 carbon atoms, polyhydric alcohols, esters of long chain fatty acids and oils in admixture with a minor proportion of a surfactant. The patentees describe the surfactants or wetting agents as necessary to compensate for the wetting resistance of the hydrophobic film to combine with the liquid surface modifier, and states that non-ionic, cationic and anionic surfactants may be used. Among the surfactants listed is sodium dioctyl sulfosuccinate, though no specific examples utilizing this surfactant are disclosed. In any event, all surfactants and wetting agents are equated, and all are used in minor amounts in relation to the liquid surface modifying agents.
Unfortunately, it has been found that the use of various viscous or sticky liquids as the predominant ingredient of coating compositions such as disclosed by McConnell et al and other patents, though improving cling values, also produces certain disadvantages results, such as imparting an unpleasant odor or taste to the film and the food wrapped therein. Moreover, the coatings frequency have a greasy feel and smeary appearance which renders them practically unacceptable as a food wrap for sale to consumers.